JPH04349615A - Formation of polycrystalline silicon thin film - Google Patents

Abstract

PURPOSE:To form on a substrate in a short time a polycrystalline silicon thin film, which has greater crystal grains and is excellent in crystal continuity, without exposing to high temperature conditions. CONSTITUTION:There is formed on a glass substrate 1 an amorphous silicon film involving fluorine atom and hydrogen atom mixed therein using mixture gas of a silane gas (SiH4) and silicon fluoride gas (SiF4). Then, the glass substrate 1 is inserted into a heating furnace where the glass substrate 1 is set to 580 deg.C and is left behind as it is for one hour to render the amorphous silicon film to solid phase growth for yielding of a porous silicon thin film 2. Thereupon, the crystal growth of silicon is promoted owing to the action of the fluoride atom and the hydrogen atom in the amorphous silicon film, so that a polycrystalline silicon thin film 3 can be formed which becomes a thin film under heating conditions in a short time (one hour) and has greater crystal grain and further which is excellent in crystal continuity.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a method for forming polycrystalline silicon thin films that can be used for thin film transistors, solar cells, etc. The present invention relates to an improvement in a method for forming a polycrystalline silicon thin film in a short time without exposing the silicon thin film to high-temperature conditions.

0002

[Prior Art] A polycrystalline silicon thin film is a crystalline silicon thin film formed by aggregating a large number of crystalline silicon particles of several hundred angstroms to several micrometers in size, and has an excellent electron mobility that is one to two orders of magnitude higher than that of amorphous silicon. In addition to these characteristics, it has the advantage that it can be formed into a film on an amorphous substrate such as glass, which is difficult to do with single-crystal silicon.

Conventionally, methods for forming this type of thin film include vapor phase growth methods such as thermal CVD and plasma CVD, in which polycrystalline silicon is directly deposited on a substrate such as glass to obtain a polycrystalline silicon thin film; A solid phase growth method or the like is used in which amorphous silicon is once formed on the substrate and then heated in a heating furnace for a long time to grow crystals to obtain a polycrystalline silicon thin film.

However, when a polycrystalline silicon thin film is obtained by the above-mentioned vapor phase growth method, as shown in FIG. There is a drawback that an ungrown region c with insufficient crystal growth is likely to be formed at the interface between the polycrystalline silicon thin film b and the substrate a, and the continuity of the crystal is lost, resulting in deterioration of electrical properties such as electron mobility. Ta.

[0005] When attempting to reduce the thickness of the polycrystalline silicon thin film formed on the substrate, the proportion of ungrown regions included in the film becomes relatively large. Another disadvantage is that it is difficult to obtain a polycrystalline silicon thin film with large crystal grain size and excellent crystal continuity.

[0006] For this reason, the solid phase growth method, which does not have the above-mentioned drawbacks, is mainly used.

[0007]

[Problems to be Solved by the Invention] When forming a polycrystalline silicon thin film using this solid phase growth method, an amorphous silicon film is first formed on a substrate a such as glass by a film deposition method such as plasma CVD. , and this was heated to generate silicon crystal nuclei within the amorphous silicon film, and the silicon crystal nuclei were grown in a solid phase to obtain a polycrystalline silicon thin film b as shown in FIG. .

The generation rate of silicon crystal nuclei is extremely slow (approximately two orders of magnitude slower) than the solid phase growth rate of crystal nuclei under temperature conditions below about 580°C. When trying to obtain a thin film, there was a problem in that it required crystallization annealing time of 10 hours or more, depending on the film thickness.

On the other hand, if the crystallization annealing temperature is set to a high temperature of 600° C. or higher for the purpose of shortening the crystallization annealing time, the rate of generation of crystal nuclei is too fast, and as shown in FIG. There is a problem that the nucleus density of the polycrystalline silicon is higher than that when the temperature is set at a low temperature (see Fig. 5B), and the crystal grain size of the obtained polycrystalline silicon becomes smaller, and the applicable substrate is limited to expensive heat-resistant substrates. There was a problem with this.

[0010] Note that boron (
A method has also been developed to accelerate the crystal growth rate of silicon by incorporating specific impurities such as B), phosphorus (P), and arsenic (As) (see "SOI Structure Formation Technology" published by Sangyo Tosho), but this method Since the polycrystalline silicon film obtained in 1. contains impurities such as electrically active boron (B), it has the drawback that it is difficult to apply it to the channel of a thin film transistor, for example, and the method has a narrow range of application. It wasn't too much.

The present invention has been made in view of these problems, and its object is to develop a polycrystalline silicon thin film, which is thin, has a large crystal grain size, and has excellent crystal continuity, under high temperature conditions. It is an object of the present invention to provide a method for forming a polycrystalline silicon thin film in a short time without exposing it to heat.

0012

[Means for Solving the Problems] That is, the present invention is based on a method of forming a thin film of polycrystalline silicon on a substrate or on an insulating film provided on this substrate, and fluorine atoms are formed on the substrate or on the insulating film. The method is characterized by forming an amorphous silicon film into which hydrogen atoms are mixed, and heating the film to form a polycrystalline silicon film.

[0013] In such technical means, glass,
Examples of means for forming an amorphous silicon film in which fluorine atoms and hydrogen atoms are mixed on a substrate such as ceramics or an insulating film such as SiOX or SiNX formed on this substrate include SiH4, Si2 H6, Si3.
Silicon hydride such as H8 and SiHm F4-m (however,
m is fluorinated silane and/or SiF represented by 1 to 3)
4. Directly forming an amorphous silicon film mixed with fluorine atoms by thermal CVD or plasma CVD using a mixed gas with silicon fluoride such as Si2 F6, or SiH4, Si2 H6, Si3 H8, etc. A method can be applied in which an amorphous silicon film is once formed by thermal CVD or plasma CVD using silicon hydride, and then fluorine atoms are introduced into this film by ion implantation.

As an example, the conditions for film formation by plasma CVD using a mixed gas of SiH4 and SiF4 are as follows: The substrate temperature is 150 to 4
00° C., and the high frequency power is set to 5 to 20 W. Note that the discharge may be a high frequency discharge, a direct current discharge, or a microwave discharge.

[0015] Furthermore, the mixing ratio of fluorine atoms and hydrogen atoms in the amorphous silicon film is arbitrary; for example, for fluorine atoms, it is about 1018 to 1021 atoms/cm3, and for hydrogen atoms, it is about 1018 to 2×1 atoms/cm3.
Set to about 0.022 pieces/cm3.

[0016]

[Operation] According to such technical means, fluorine atoms and hydrogen atoms are contained in the amorphous silicon film formed, and the crystal growth of silicon is promoted by the action of these fluorine atoms and hydrogen atoms. It becomes possible to form a polycrystalline silicon thin film under low temperature and short heating conditions.

The inventor of the present invention conjectures that the fluorine atoms and hydrogen atoms in the amorphous silicon film promote silicon crystal growth as follows.

That is, fluorine atoms and hydrogen atoms in the amorphous silicon film combine to form hydrogen fluoride (HF) during crystallization annealing, and molecules of this hydrogen fluoride are released from the amorphous silicon film. The silicon dangling bonds generated at this time recombine to form a crystal lattice to lower the potential energy, and the repetition of the detachment of the hydrogen fluoride molecules and the recombination of the dangling bonds results in crystallization. This seems to be due to the promotion of nucleus generation and solid phase growth.

[0019]

Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

First, an amorphous silicon film having a thickness of 1000 angstroms and containing a large amount of fluorine atoms and hydrogen atoms was formed on a glass substrate 1 using a plasma CVD apparatus as shown in FIG. That is, silane gas (SiH4
) and silicon fluoride gas (SiF4) 16 are introduced, and plasma discharge is generated between the electrodes 12 and 13 by the high frequency power source 15 to decompose the silane gas and the silicon fluoride gas 16, and these mixed gases are transferred to the electrodes. The amorphous silicon film having a thickness of 1000 angstroms as described above was formed by forming a film on the glass substrate 1 which was heated to about 200 to 300° C. using a heater in the device 13.

The concentration of fluorine atoms in the amorphous silicon film thus formed was 1019 atoms/cm3, and the concentration of hydrogen atoms was 2.5×1021 atoms/cm3.

Next, the glass substrate 1 on which the amorphous silicon film was formed was placed in a heating furnace set at 580° C. and left for one hour to grow an amorphous silicon film in a solid phase as shown in FIG. Polycrystalline silicon thin film 2
I asked for

At this time, silicon crystal growth is promoted by the action of fluorine atoms and hydrogen atoms in the amorphous silicon film, so that a thin film with a large crystal grain size can be formed under the above-mentioned short time (1 hour) conditions. It became possible to form a polycrystalline silicon thin film 3 with excellent crystal continuity.

As a comparative example, the above silane gas (SiH
4) Amorphous silicon film formed using only
When a film containing no fluorine atoms was grown in a solid phase under the same temperature conditions, the crystallization annealing time required 15 hours.

[0025] Also, for the amorphous silicon film mixed with fluorine atoms and hydrogen atoms, the temperature of the heating furnace was set to 5.
When a similar solid-phase growth process was performed at a temperature of 40° C., the required crystallization annealing time was 5 hours.

On the other hand, as a comparative example, an amorphous silicon film containing no fluorine atoms was prepared under the same temperature conditions (540
℃), the crystallization annealing time required 50 hours.

[0027]

Effects of the Invention According to the present invention, crystal growth of silicon is promoted by the action of fluorine atoms and hydrogen atoms in the amorphous silicon film, so that a polycrystalline silicon thin film can be formed under low temperature and short heating conditions. have.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a polycrystalline silicon thin film according to an example.

FIG. 2 is an explanatory diagram of a configuration of a plasma CVD apparatus applied in an example.

FIG. 3 is a cross-sectional view of a polycrystalline silicon thin film formed by a conventional vapor phase growth method.

FIG. 4 is a cross-sectional view of a polycrystalline silicon thin film formed by a conventional solid phase growth method.

FIGS. 5A and 5B are diagrams explaining the principle of conventional solid phase growth.

[Explanation of symbols]

1 Glass substrate 2 Polycrystalline silicon thin film

Claims (1)

[Claims] 1. A method for forming a thin film of polycrystalline silicon on a substrate or on an insulating film provided on the substrate, wherein an amorphous silicon film mixed with fluorine atoms and hydrogen atoms is formed on the substrate or the insulating film. 1. A method for forming a polycrystalline silicon thin film, the method comprising: forming a polycrystalline silicon thin film, and heating the polycrystalline silicon film to form a polycrystalline silicon film.